Partial thromboplastin time

The partial thromboplastin time (PTT) or activated partial thromboplastin time (aPTT) is a performance indicator measuring the efficacy of both the intrinsic and the common coagulationpathways. Apart from detecting abnormalities in blood clotting, it is also used to monitor the treatment effects with heparin, a major anticoagulant.

Methodology

The PTT is measured by using a citratedsample, which arrests coagulation by binding calcium. In order to activate the intrinsic pathway, phospholipidand another activator are mixed into the plasmasample (such as silica, celite, kaolin, ellagic acid), and calcium(to revert the anticoagulant effect of the citrate). The time is measured until a thrombus(clot) forms.

The test is termed "partial" due to the absence of tissue factorfrom the reaction mixture.

Interpretation

Values below 25 secondsand over 39 s (depending on local normal ranges) are generally abnormal. Shortening of the PTT has little clinical relevance, as most thrombosispatients have normal coagulation studies. Prolonged APTT may indicate:

• use of heparin(or contamination of the sample)
• antiphospholipid antibody(especially lupus anticoagulant, which paradoxically increases propensity to thrombosis)
• coagulation factor deficiency (e.g. hemophilia)

To distinguish the above causes, mixing studies are performed, in which the patient's plasma is mixed (initially at a 50:50 dilution) with normal plasma. If the abnormality does not disappear, the sample is said to contain an "inhibitor" (either heparin, antiphospholipid antibodies or coagulation factor specific inhibitors), while if it does correct a factor deficiency is more likely. Deficiencies of factors VIII, IX, XIand XIIand rarely von Willebrand factor(if causing a low factor VIII level) may lead to a prolonged aPTT correcting on mixing studies.

External link

• aPTT testingde:Partial Thromboplastin Time

fr:Temps de céphaline activé

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It uses material from the http://en.wikipedia.org/wiki/Partial+thromboplastin+time Wikipedia article Partial thromboplastin time.